The contaminants are so named solely because they are undesired. If one were trying to culture Penicillium
and spores of a Psilocybe
settled onto the agar media and germinated, the resulting mycelia would be the so-called "contaminant." The contaminants in mushroom culture, however, are primarily molds, bacteria, viruses and insects. The pathway by which a disease is introduced, known as the vector
of contamination, can be used to trace the contaminant back to its site of origin using simple deduction. By observing how a contaminant affects the mushroom crop and by carefully noting the conditions in which it flourishes, a cultivator can soon identify its cause.
The five most probable vectors of contamination are:
1. the cultivator.
2. the air.
3. the substrate to be inoculated.
4. the spores or mycelium that was being transferred.
5. the inoculating tools, equipment, containers, facilities, etc.
Different contaminants are associated with different stages of mushroom cultivation.
Contaminants in agar culture most often come from airborne spores. Grain cultures contaminate from airborne spores and from a source which many cultivators fail to identify: the grain used in spawn making which is laden with spores of imperfect fungi, yeasts and bacteria.
Molds and bacteria do not grow well in a climate specifically adjusted for mushrooms. Although both mushrooms and contaminants prefer humid conditions, the latter thrive in prolonged stagnant air environments whereas mushrooms do not. The differences are frequently subtle—amounting to only a few percentage points in relative humidity and slight adjustments to the air intake dampers in the growing room.
The contaminants can be divided into two well defined groups. Those attacking the mushrooms are called pathogens
while those competing for the substrate are labeled indicators
(Mushroom pathogens are either molds, bacteria, viruses or pests; indicators are always fungi of some sort). In general, mushroom pathogens are not as numerous as the competitor molds, though they can be much more devastating.
Source: Paul Stamets and J.S.Chilton: The Mushroom Cultivator